CVT control method

ABSTRACT

For a CVT it is proposed to determine an operating point from the number of revolutions of the primary disc (12) and the number of revolutions of the secondary disc (13). The operating point is assigned to a first characteristic field which has ranges of unauthorized ratios and one range of authorized ratio. If an error occurs, the pressure level in the adjustment area of the secondary disc (pSEK) is increased in a first step. If the error continues an emergency program is activated in a second step.

BACKGROUND OF THE INVENTION

The invention concerns a method for control of a CVT in which anelectronic control unit detects and monitors the speed signals of aprimary and a secondary disc.

The methodical operation of CVT is usually monitored by an electroniccontrol unit. EP-PS 0228884 thus proposes to monitor the methodicaloperation of two speed sensors assigned to the primary and secondarytaper disc pairs. Hereby is tested whether or not the speed signal ispresent. In the absence of speed of the secondary taper disc pair, it isproposed to adjust the reduction ratio to a fail-safe value. In theabsence of the speed of the primary disc pair, it is proposed that thespeed of the primary taper disc pair be adjusted to a fail-safe value.

The above described prior art has the disadvantage that brief failuresare interpreted as an absence of the speed signal.

SUMMARY OF THE INVENTION

In view of the above, the invention is to solve the problem of providingfor a CVT, a method which detects brief failures of the speeds of theprimary and the secondary disc pairs and reacts thereto with adequateflexibility.

According to the invention, the problem is solved by the fact that inthe presence of a stationary state while driving, a ratio of thevariator is determined from the speed signals of the primary andsecondary discs, the ratio is assigned to a first characteristic field,said characteristic field having one range of authorized ratios andranges of unauthorized ratios. An error is detected when the ratio is inan unauthorized range and with detection of the error the pressure levelof the variator is increased in a first step and if the error continuesan emergency program is activated in a second step.

The solution, according to the invention, offers the advantage that byassigning the ratio to ranges of authorized and unauthorized ratio, notonly is there tested the presence of speed signals per se, but also thecorrectness thereof in the stationary and dynamic ranges. If it is foundthat the ratio lingers in an unauthorized range, in adequate flexiblereaction thereto the level of contact pressure of the variator isincreased. In case the error continues, an emergency drive program isactivated in a second step.

In a development of this, it is proposed that the range of authorizedratios has as upper limit the highest possible ratio of the variator andas lower limit the minimum possible ratio of the variator and, indevelopment of this, it is proposed that the characteristic field has anadditional range of stoppage, an error within this range existing whenthe number of revolutions of the primary disc is higher than the productof the number of revolutions of the secondary disc multiplied by themaximum possible ratio of the variator.

In a development, it is proposed that in the absence of error during adynamic operation, an actual gradient of the ratio be determined fromthe ratio of the variator at a first and at a second moment. Thereafter,an operating point is determined from the actual gradient and the numberof revolutions of the secondary disc at the second moment. Thisoperating point is assigned to a second characteristic field, whereinthe second characteristic field has a range of authorized and a range ofunauthorized operating points. An error is detected when the operatingpoint is in the range of unauthorized operating points wherein, with thedetection of the error, in a first step the contact pressure level onthe variator is increased and if the error continues, an emergency driveprogram is activated in a second step. This development offers theadvantage that after it having been found, via the first characteristicfield, that an operating point is in the authorized range, it isadditionally tested, via the second characteristic field, whether thedynamics of adjustment of the variator coincide with predeterminedvalues. In other words, by said additional function is tested whetherslip occurs on the variator in the dynamic operation. In CVT of thetaper disc continuously variable type, a slip between the taper discsand the continuously variable member is known to result in damage of thecontinuously variable member. The same applies to a toroidal CVT.

In development of this, it is proposed that in the absence of error, theactual gradient be compared with the theoretical gradient and therefrom,by difference formation a divergence be detected which is thensubsequently processed.

This development offers the advantage of there being detected lingeringdivergences, i.e. which have an effect upon the adjustment dynamics.

BRIEF DESCRIPTION OF THE DRAWING(S)

A preferred embodiment is shown in the drawings wherein:

FIG. 1 is a system diagram;

FIG. 2 is a first characteristic field;

FIG. 3 is a second characteristic field;

FIG. 4 is a flow chart;

FIG. 5 is a flow chart for detecting stoppage; and

FIG. 6 is a flow chart for testing according to the secondcharacteristic field.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1 shows a reduced system of diagram of a CVT of the taper disccontinuously variable type. The complete system was disclosed in ATZAutomobiltechnische Zeitschrift 96 1994) 6, page 380. A variator isshown with reference numeral 1. It comprises a primary taper disc pair 2located on the input side, a continuously variable member 4 and asecondary disc pair 3 located on the output side. Each taper disc pairconsists of one taper disc stationary in axial direction and one taperdisc movable in axial direction. By the axial position of the movabletaper disc is determined the spin radius of the continuously variablemember and thus the ratio. An electronic control unit 5 receives asignal from a selector lever 10, a speed signal of the primary disc 12,a speed signal of the secondary disc 13 and input parameters 11. Inputparameters are, e.g. the signal of a throttle valve, the temperature ofthe hydraulic medium, etc. From said input parameters, the electroniccontrol unit 5 determines the operating parameters of the CVT, e.g theratio, the operating point and the pressure level in the primary andsecondary discs. By means of an electromagnetic pressure regulator 6,the electronic control unit 5 determines the pressure level in theadjustment area of the primary taper disc pair and by a pressureregulator 7, the pressure level in the adjustment area of the secondarytaper disc pair. Hereby is established, via the pressure level in theadjustment area of the primary disc, an input speed of the CVT, i.e. thespeed of an internal combustion engine. By the pressure level of thesecondary disc is determined the contact pressure continuously variablemember/secondary disc pair 3, thus the torque-transmitting capacity ofthe system. Pressure is supplied to the two pressure regulators 6 and 7by a pump 9 which conveys the hydraulic medium from an oil pan 8 via afilter without reference numeral.

FIG. 2 shows a first characteristic field. On the abscissa are plottedspeed values of the secondary disc (nS2) and on the ordinate speedvalues of the primary disc (nS1). The characteristic field containsthree ranges: ranges I and III represent ranges of unauthorizedoperating points and range 11 represents a range of authorized operatingpoints. A line 14 forms the upper limit of range 1. A line 15 forms thelower limit of range III. The maximum possible ratio range of thevariator is shown by the lines 16 and 17. In other words, within saidtwo lines 16 and 17 the ratio of the variator can be changed. As can beseen from FIG. 2, the line 16 does not coincide with the line 14 nor theline 17 with the line 15. The range of authorized ratios is formed fromthe lines 15 and 14. In FIG. 2 is additionally shown with referencenumeral 18 a dotted line that delimits the range of stoppage. The rangeextends thus from the zero point to the abscissa value nS2₋₋ min. Theoperating points of the first characteristic field are determined bycalculating, in the presence of a stationary state while driving(nS2>limit value), from the speed signals of the primary disc (nS1) andsecondary disc (nS2) a ratio of a variator (iV=nS1/nS2). In FIG. 2 isshown an operating point P1 with the appertaining speed values of theprimary disc (nS1) and the secondary disc (nS2). In this example, theoperating point P1 is in the range II, thus in the authorized range. Theoperating point P2 is in the range I, i.e. a range of unauthorizedoperating points. This operating point is detected as error. With thedetection of the error, the contact pressure level of the variator isincreased in a first step. If the operating point continues in anunauthorized range, an emergency drive program is activated in a secondstep. Such an emergency drive program is known, e.g. from DE-OS 44 36506.

In FIG. 3 is shown a second characteristic field. This secondcharacteristic field is used when it resulted form the test of theoperating point, according to the first characteristic field of FIG. 2,that the operating point lies in the authorized range I. In this secondcharacteristic field, speed values of the secondary disc (nS2) are shownon the abscissa and values of the gradient of ratio of the variator(iV₋₋ GRAD) on the ordinate. This second characteristic field shows arange I which represents the range of unauthorized operating points. Therange II represents the range of authorized operating points. Theoperating points are detected by determining during a dynamic operation,from the ratio of the variator at a first and at a second moment, anactual gradient of the ratio. The operating point is then establishedfrom this actual gradient and the speed of the secondary disc at thesecond moment. In FIG. 3 are shown three examples, namely, operatingpoints P1, P2 and P3. All three operating points have here the sameabscissa value, namely, nS2(t2). The operating point P3 results from theabscissa value and the gradient of the change of ratio (iV-GRAD) withthe value (3). The operating point P3 is herein the range I, i.e. theunauthorized operating range. This case arises when slip occurs on thesystem variator/continuously variable member. As reaction to the factthat the operating point is in the unauthorized range, the contactpressure level on the variator is increased. If the operating point P3continues in the unauthorized range I, an emergency drive program isactivated in a second step. The operating point P2 is on the dividingline between the ranges I and II, ordinate value (2). This operatingpoint P2 is in the authorized range. Likewise, the operating point P1 isin the authorized range, ordinate value (1). From this secondcharacteristic field, it is further possible to deduce whether theadjustment gradient of the variator, i.e. the actual value, divergesfrom a theoretical value. The theoretical value of the adjustmentgradient can be predetermined, e.g. by the driving strategy. Thedivergence can be determined by difference formation and thensubsequently processed, e.g. as control divergence for an added controlcircuit or a limitation of the driving strategy dynamics. Theintroduction of the additional second characteristic field thus offersthe advantage of it being possible to detect slip occurring in thedynamic operation within the authorized range of the firstcharacteristic field. By comparison of theoretical and actual values ofthe adjustment dynamics can also be detected unauthorized drivingstrategy standards, etc., e.g. control divergence, leakage.

In FIG. 4 is shown a flow chart which is part of a main program. Thelatter starts with step S1 with the reading in of the number ofrevolutions of the primary disc (nS1) and the number of revolutions ofthe secondary disc (nS2). In step S2 is tested whether the number ofrevolutions of the secondary disc is < a limit value, nS2₋₋ MIN here.This corresponds to the range of stoppage of FIG. 2. In case of apositive result, i.e. the number of revolutions of the secondary disc islower than this limit value, the program branches to point A which isexplained in relation to FIG. 5. In case of negative result, with stepS3 follows the calculation of the ratio of the variator iV, i.e theactual operating point. In step S4 is tested whether the ratio of thevariator is in the authorized range. This interrogation corresponds tothe test of the operating point by means of the first characteristicfield of FIG. 2. If it is established in step S4 that the operatingpoint is in the authorized range, the program branches to point Bexplained in relation to FIG. 6. In case of negative result, i.e. theoperating point is an unauthorized range, the pressure level in theadjustment area of the secondary disc (pSEK) is increased in a firststep in step S5. The pressure level, in the adjustment area of thesecondary disc (pSEK), is known to be decisive for thetorque-transmitting capacity of the variator. By increasing the pressurelevel is sought to eliminate the previously found slip. In step S6 istested whether the operating point is now in the authorized range. Incase of positive result, in step S9 a contact pressure correction, inthe form of a pressure adaptation, is stored over the transmission lifecycle, branching to point B. If the result is negative, i.e. a slipcontinues to be found, with step S7 is activated an emergency driveprogram such as known, e.g. from DE-OS 44 36 506. In step S8, theprogram terminates and a return to the main program follows.

FIG. 5 shows a partial program, starting with the branching point A. Instep S1 is tested whether the number of revolutions of the primary disc(nS1) is lower than the product from the number of revolutions of thesecondary disc (nS2) multiplied by the maximum possible ratio of thevariator (i₋₋ HIGH). In case of positive result, with step S2 the mainprogram is returned to. In case of negative result, i.e. slip exists inthe variator, in step S3, in a first reaction, the pressure level in theadjustment area of the secondary disc (pSEK) is increased. In step S4,again it is tested whether the number of revolutions of the primary disc(nS1) is lower than the previously described product. In case ofpositive result, i.e. the slip has been eliminated, in step S5 theincrease of the contact pressure is stored as adaptive function over thetransmission life cycle. Thereafter the main program is returned to withstep S6. In case of negative result, i.e. a slip persists, an emergencydrive program is activated in step S7 and, with step S8, the mainprogram is returned to.

FIG. 6 shows a flow chart, which describes the operation of the secondcharacteristic field, according to FIG. 3. The program starts with pointB. In step S1 is read in the ratio of the variator (iV) at a firstmoment (t1) and at a second moment (t2). In step S2, the gradient(iV-GRAD) is calculated from said two values. In step S3, the operatingpoint is then established. Here the operating point is a function of thenumber of revolutions of the secondary disc at the second moment (t2)and of the previously calculated gradient of the variator ratio(iV-GRAD). In step S4 is tested whether the operating point is in theauthorized range. In case of negative results, i.e. it lies, accordingto FIG. 3, in the range I, the steps S5, S6, S7, S8 and S13 are runthrough. In case of positive result, the steps S9, S10, S11 and S12 arerun through. If it is found in Step S4 that the operating point is notin the authorized range, as reaction hereto the pressure level in theadjustment area of the secondary disc (pSEK) is increased in step S5.Thereafter, in step S6, it is again tested whether the operating pointis now in the authorized range, i.e. whether the slip has beeneliminated. In case of positive result, in step S13 the increase of thecontact pressure is stored as dynamic contact pressure adaptation. Thenit is proceeded with step S8, namely, the return to the main program. Incase of negative result, i.e. the operating point still is not in theauthorized range, the emergency operation is activated with step S7. Incase of positive result in step S4, it is tested in step S9 whether theactual value of the gradient exactly corresponds to a theoretical value.In case of positive result, the main program is returned to with stepS10. In case of negative result, the difference between actual andtheoretical values is subsequently processed with step S11 and then,with step S12 the main program is returned to. The difference found instep S11 can be used to detect a lingering divergence and subsequentlyprocess it in the sense of a control divergence. The method, accordingto the invention, can be applied not only to CVT's of the taper disctype but also to CVTs of the toroidal type.

    ______________________________________                                        Reference numerals                                                            ______________________________________                                                1 variator                                                                    2 primary taper disc pair                                                     3 secondary taper disc pair                                                   4 continuously variable member                                                5 electronic control unit                                                     6 electromagnetic pressure regulator                                          7 electromagnetic pressure regulator                                          8 oil pan                                                                     9 pump                                                                       10 selector lever                                                             11 input parameters                                                           12 speed of primary disc                                                      13 speed of secondary disc                                                    14 limit of range I                                                           15 limit of range II                                                          16 iV.sub.-- HIGH                                                             17 iV.sub.-- LOW                                                              18 limit of stoppage range                                             ______________________________________                                    

What is claimed is:
 1. A method for control of a CVT in which an electronic control unit (5) detects and monitors a speed signal (12) of a primary disc (nS1) and a speed signal (13) of a secondary disc (nS2), said electronic control unit (5) determining, via electromagnetic servo components (6, 7) and hydraulic valves, the pressure level in an adjustment area of said primary disc and in an adjustment area of said secondary disc (pSEK), characterized in that in a stationary state while driving (nS2>limit value), a ratio of a variator (iV=nS1/nS2) is determined from the speed signals of said primary (nS1) and secondary (nS2) discs, the ratio (iV) is assigned to a first characteristic field has one range of authorized and ranges of unauthorized ratios (iV), an error is detected when the ratio (iV) is in an unauthorized range wherein with the detection of the error the contact pressure level of the variator is increased in a first step and if the error continues, an emergency drive program is activated in a second step.
 2. The method according to claim 1, characterized in that said range of authorized ratios has as upper limit the highest possible ratio of said variator (iV₋₋ HIGH) and as lower limit the minimum possible ratio of said variator (iV₋₋ LOW).
 3. The method according to claim 2, characterized in that said first characteristic field has an additional range of the stoppage (nS2<limit value) and an error exists when the speed of said primary disc (nS1) is higher than the product from the number of revolutions of said secondary disc (nS2) multiplied by the maximum possible ratio of said variator (iV₋₋ HIGH).
 4. The method according to claim 1, characterized in that in the absence of error, during a dynamic operation, from the ratio of said variator (iV) at a first (t1) and a second (t2) moment, an actual gradient of the ratio (iV₋₋ GRAD₋₋ IST) is determined from the actual gradient (iV₋₋ GRAD₋₋ IST) and the number of revolutions of said secondary disc at the second moment (nS2(t2)) an operating point is determined, said operating point is assigned to a second characteristic field, said second characteristic field has one range of authorized and one range of unauthorized operating points, an error being detected when the operating point is in the range of unauthorized operating points and with the detection of the error, the contract pressure level of said variator is increased in a first step and if the error continues, an emergency drive program is activated in a second step.
 5. The method according to claim 4, characterized in that in the absence of error, the actual gradient of the ratio (iV₋₋ GRAD₋₋ IST) is compared with a theoretical gradient of the ratio (iV₋₋ GRAD₋₋ SOLL) and therefrom a divergence is detected by difference formation and the divergence is subsequently processed. 